fix static_functions_not_mutating error msg

Author: felixinkinto

include/swift/AST/DiagnosticsSema.def

@@ -1735,7 +1735,10 @@ ERROR(functions_mutating_and_not,none,
       "method must not be declared both %0 and %1",
       (SelfAccessKind, SelfAccessKind))
 ERROR(static_functions_not_mutating,none,
-      "static functions must not be declared mutating", ())
+      "%0 modifier cannot be used on static functions",
+      (SelfAccessKind))
+NOTE(static_functions_not_mutating_detail,none,
+      "static members cannot access instance properties or self", ())
 
 ERROR(readwriter_mutatingness_differs_from_reader_or_writer_mutatingness,none,
       "%0 cannot be %1 when "

lib/Sema/TypeCheckAttr.cpp

@@ -785,8 +785,10 @@ void AttributeChecker::visitMutationAttr(DeclAttribute *attr) {
     }
   }
   // Verify that we don't have a static function.
-  if (FD->isStatic())
-    diagnoseAndRemoveAttr(attr, diag::static_functions_not_mutating);
+  if (FD->isStatic()) {
+    diagnoseAndRemoveAttr(attr, diag::static_functions_not_mutating, attrModifier);
+    diagnose(FD, diag::static_functions_not_mutating_detail);
+  }
 }
 
 void AttributeChecker::visitDynamicAttr(DynamicAttr *attr) {

test/Sema/immutability.swift

@@ -131,7 +131,8 @@ struct SomeStruct {
 
   static let type_let = 5  // expected-note {{change 'let' to 'var' to make it mutable}} {{10-13=var}}
 
-  mutating static func f() {  // expected-error {{static functions must not be declared mutating}} {{3-12=}}
+  mutating static func f() {  // expected-error {{'mutating' modifier cannot be used on static functions}} {{3-12=}}
+                              // expected-note@-1 {{static members cannot access instance properties or self}}
   }
 
   mutating func g() {

test/decl/overload.swift

@@ -410,7 +410,8 @@ protocol ProtocolWithMutating {
   mutating func test2(_ a: Int?) // expected-note {{previously declared}}
   func test2(_ a: Int!) // expected-error{{invalid redeclaration of 'test2'}}
 
-  mutating static func classTest1() // expected-error {{static functions must not be declared mutating}} {{3-12=}} expected-note {{previously declared}}
+  mutating static func classTest1() // expected-error {{'mutating' modifier cannot be used on static functions}} {{3-12=}} expected-note {{previously declared}}
+                                    // expected-note@-1 {{static members cannot access instance properties or self}}
   static func classTest1() // expected-error{{invalid redeclaration of 'classTest1()'}}
 }
 

test/decl/subscript/static.swift

@@ -20,8 +20,10 @@ func useMyStruct() {
 
 struct BadStruct {
   static subscript(_ i: Int) -> String {
-    nonmutating get { fatalError() } // expected-error{{static functions must not be declared mutating}}
-    mutating set { fatalError() } // expected-error{{static functions must not be declared mutating}}
+    nonmutating get { fatalError() } // expected-error {{'nonmutating' modifier cannot be used on static functions}}
+                                     // expected-note@-1 {{static members cannot access instance properties or self}}
+    mutating set { fatalError() } // expected-error {{'mutating' modifier cannot be used on static functions}}
+                                  // expected-note@-1 {{static members cannot access instance properties or self}}
   }
 }